The invention relates to a heating device for a plant for hot forming blanks and to a corresponding plant and method for hot forming blanks.
The hot forming of metal sheets is a relatively new development trend in the component manufacturing, in particular for vehicle bodies. In the context of this application, following the well-established language use in the field of shaping technology, metal sheets used hereby are accordingly also identified as “blanks”. In principle, a blank is a correspondingly cut, die cut, joined and/or preformed metal sheet.
The hot forming makes it possible to produce components comprising a high stability and a complex geometry without resilience and allows for a significant weight reduction in the case of the auto bodies manufactured therewith, e.g., as well as for an increase of safety, for example of passengers of a corresponding vehicle.
With the increasing demands of stability and stiffness of structure components, in particular in the vehicle, high-strength and highest-strength steels are used increasingly. An increase of the stability provides for a reduction of the vehicle weight, which provides in particular for a reduced pollutant emission and fuel consumption. In the case of current vehicle models, the use of hot formed components can save up to 25 kg of weight.
In essence, hot forming methods are combined forming, hardening and tempering techniques. By using corresponding steels, such as manganese-boron steels, for example, stabilities of more than 1,500 MPa can be reached therewith. Press-hardening methods comprise, for example, the heating of blanks to a temperature, which lies above the complete austenitization temperature, e.g. above 850° C., and the subsequent quick cool-down of the blank in the tool. The desired martensitic structure comprising the desired stability is formed through this. The combination of the forming with the quenching in a tool is occasionally also identified as press- or form-hardening.
In principle, so-called roller hearth furnaces are used for preheating the blanks in response to the hot forming of highest-strength materials for car bodies. The heating of such furnaces typically takes place by means of steel pipes, which are heated electrically or by means of gas burners. To attain process cycle times, which are as short as possible, a certain “supply” of preheated components is necessary in the plant. The heat treatment duration for the temperature control of the steel represents a significant parameter, which defines the clock cycle of a corresponding press. However, due to the low degree of efficiency at temperatures of below 600°, the efficiency of roller hearth furnaces is small. Roller hearth furnaces encompass a length of up to 50 meters and thus require corresponding structural conditions, including an efficient dissipation of excess heat. Drum melting furnaces, which are used as an alternative to roller hearth furnaces, to preheat components, also encompass corresponding disadvantages. They are also heated by means of steel pipes and are unsatisfactory in view of their degree of efficiency.
Press-hardened components are characterized by their high stability and stiffness. As mentioned, metal sheet thicknesses can be reduced through this and weight can thus be saved. However, the low ultimate strain of press-hardened components is problematic, which can lead to the formation of tears in the case of subsequent production operations, such as the welding of further parts, e.g. For this reason, it is desirable to embody certain areas of a vehicle body, e.g. so as to be press-hardened, and to embody other areas such that they encompass a higher ductility and can thus absorb more energy by means of plastic deformation.
Current approaches which are used to generate such locally different characteristics, so-called “tailored properties”, include the specific influencing of alloy elements of corresponding semifinished parts, the manufacture of so-called “tailored welded blanks”, thus blanks, which are joined from different materials, the partial (local) heating by means of inductive or conductive heating technologies, the partial temperature control of certain areas of the press-hardening tools by locally heating, the partial tempering of the press-hardened components and the masking of certain component areas, so as to suppress the heating (and thus the austenitization) in a corresponding roller hearth furnace. However, such methods are extensive, the result thereof is oftentimes unsatisfactory, and oftentimes cause excessive costs.
There is thus a need for improved possibilities to provide blanks comprising locally different characteristics.